The present exemplary embodiment relates to a method and apparatus for calibrating a thermistor. It finds particular application in conjunction with thermistors used in liquid drop ejectors such as print heads operative to emit phase change ink, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
By way of background, thermistors are often used on print heads to control temperature. In an effort to reduce cost and improve temperature control response, thermistors are deposited and patterned on a print head using thin-film or thick-film processes. Such a configuration is disclosed in co-pending and commonly assigned U.S. application Ser. No. 10/990,110, filed Nov. 15, 2004, entitled “Thin Film and Thick Film Heater and Control Architecture for a Liquid Drop Ejector” and having Michael Young et al. named as inventors, which application is hereby incorporated by reference herein.
When thin film fabrication is implemented, however, process variations of +/−10% in the sheet resistance and the temperature coefficient of resistance (TCR) are typical. This can result in large temperature measurement errors. Of course, accuracy and precision in temperature measurement is important for print head control and maintenance. Thus, to overcome this potential measurement error, every thermistor fabricated using thin-film techniques should be calibrated at the operating temperature of the print head. To do so, each print head must be brought to its operating temperature by external heating means. This may include the use of a heated chuck and a heated air gun for example. Other methods may include adjusting a heater control unit until a proper drop mass is achieved. These approaches, though, result in excessive cost in terms of time and resources. Therefore, a low cost calibration method is needed.
Likewise, because of the thin film fabrication variations of +/−10% in sheet resistance and temperature coefficient of resistance (TCR), thin film thermistors may also need to be trimmed. The current industry method of trimming thin film thermistors is expensive. Therefore, a simple low-cost and manufacturable method for trimming thin-film thermistors is desired.
A variety of prior patents address calibrating and trimming thermistors. For example, U.S. Pat. No. 5,881,451, entitled “Sensing the temperature of a printhead in an ink jet printer,” issued on Mar. 16, 1999. This patent describes a temperature compensation method for TCR variation but it assumes that TCR variation data is available for trimming.
U.S. Pat. No. 5,315,316, issued May 24, 1994, is entitled “Method and Apparatus for Summing Temperature Changes to Detect Ink Flow.” This patent describes a print head temperature control circuit which includes a temperature sensor formed on the printhead substrate. The ink drop is measured and temperature is adjused to the correct ink drop.
U.S. Pat. No. 5,075,690, issued Dec. 24, 1991 and entitled “Temperature Sensor for an Ink Jet Print Head” describes an analog temperature sensor. A more accurate response is achieved by forming the thermistor on the printhead substrate of the same polysilicon material as the heaters. The thermistor is calibrated and trimmed at the operating temperature.
U.S. Pat. No. 5,428,206, issued Jun. 27, 1995 and entitled “Positive Temperature Coefficient Thermistor Heat Generators” describes the implementation of a positive temperature coefficient (PTC) thermistor in parallel with a series combination of a high output foil heater and a bimetal switch. The bimetal switch opens when a desired temperature is reached. The positive temperature coefficient (PTC) thermistor purportedly maintains the heated object's temperature when the foil heater is switched off.
U.S. Pat. No. 5,406,361, entitled “Circuit for controlling Temperature of a Fuser Unit in a Laser Printer,” issued on Apr. 11, 1995. This patent describes a circuit which compares first reference voltage, associated with an operational temperature (200C), and second reference voltage, associated with a standby temperature (150C), to a sensor dependent voltage for laser printer fuser temperature regulation. The sensor measures the temperature of the laser printer's fuser unit and this information was used to control the fuser heating unit. Negative temperature coefficient (NTC) thermistors are used. The circuit uses analog comparators and thus does not involve using A/D conversion and extra CPU clock cycles.